1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, to an advanced design of gray rod control assemblies (GRCAs).
2. Background Information
The power produced by the reactor of a nuclear power plant is generally controlled by raising or lowering control rod assemblies within the reactor core, and the change in reactor power output required in order to accommodate a change in the demand for electrical output from the electrical power plant is commonly referred to as load follow. As described, for example, in U.S. Pat. No. 4,079,236, load follow presents many operating issues. For instance, in a pressurized water reactor (PWR) during load follow, reactivity must be controlled and axial power distribution changes in the core in response to the power level change, must be addressed.
The fuel assemblies of modern reactor cores typically employ two types of rod control assemblies to control reactivity, rod cluster control assemblies (RCCAs) and gray rod control assemblies (GRCAs). Both consist of a plurality of neutron-absorbing rods fastened at their top ends to a common hub or spider assembly. The body of the rods generally comprises a stainless steel tube which encapsulates a neutron-absorbing material, such as a silver-indium-cadmium absorber material, and the rods are slid within tubular guide thimble tubes of the fuel assembly with a control drive mechanism near the top of the spider assembly operating to control the movement of the rods within the thimble tubes. In this manner, the controlled insertion and extraction of the rods generally controls the amount of power produced by the reactor. Additionally, GRCAs are used during full power operation as described below.
Typically, GRCAs are used in load follow maneuvering because they are comprised of reduced worth control rods, commonly referred to in the art as “gray” rods, and have less impact on core distribution. The term “gray” as used herein relates to the neutron absorption characteristic of the absorber rods, and refers to the fact that the GRCA is intended to absorb only a fraction of the thermal and epithermal energy neutrons entering the absorber, as opposed to “black” control rods, which are intended to absorb a large majority of such neutrons in order to shut down the reactor. Gray rods are known to provide a mechanical shim (MSHIM) reactivity control mechanism as opposed to a chemical shim, which requires changing the concentration of soluble boron in the reactor coolant. Thus, the use of gray rods minimizes the need for processing the primary reactor coolant on a daily basis and, therefore, greatly simplifies operations. This reactivity control function is used for both full power operations and during load follow. More specifically, an existing GRCA design consists of 24 rodlets fastened at their top ends to the spider. Of the 24 rodlets within the cluster, only four rods are absorber rods, and the neutron-absorber material encapsulated within the absorber rods typically consists of an alloy containing about 80% silver (Ag), about 15% indium (In), and about 5% cadmium (Cd). Such a design poses several disadvantages.
Among the disadvantages of known Ag—In—Cd GRCA designs, is the fact that indium and cadmium have relatively large neutron cross-sections, which result in their depletion over a relatively short period of time. As a result, the rod worth of such a GRCA design is reduced below an acceptable value within about five to ten years, depending on the design and amount of usage. Continued use beyond this time results in further depletion, and the GRCAs will eventually become ineffective at controlling the reactor during load follow or providing reactivity control at full power. This undesirably leads to frequent GRCA replacement. A second disadvantage relates to changes in the local rod power for fuel rods which are adjacent to the four guide thimbles that contain the absorber rods. Specifically, because the absorber material is localized to four rodlets in known designs, a relatively large change in power, commonly referred to as the delta-power of the fuel rods, occurs, for example, during a rod pull. A rod pull is the process of extracting the GRCA from the fuel assembly. In known GRCA designs, a rod pull can result in a delta-power spike in adjacent fuel rods that may be larger than allowable limits which indicate an increased risk of fuel failure. GRCA designs which are variations of known designs, and which incorporate lesser amounts of Ag—In—Cd evenly distributed among all 24 rodlets can mitigate this problem. However, such designs will also deplete at a significantly higher rate due to lower self shielding of the indium and cadmium, and will become depleted below the acceptable rod worth in less than five years. In addition, absorber swelling due to irradiation induced transmutation in silver alloy control rod designs has been a longstanding problem in the industry for many years. Specifically, exposure of silver and indium to neutron radiation results in the formation of significant amounts of cadmium and tin, which can lead to swelling due to changes in the material density. Too much swelling of the absorber can result in the absorber contacting and potentially cracking the cladding surrounding it.
The reduced worth gray rods are typically intended to have a reactivity worth significantly lower than standard (or black) RCCA control rods that are used to shut down the reactor or provide gross reactivity control capability. The targeted reactivity worth of a gray control rod may vary depending on the application and intended operating strategy of the plant. Further, the weight of a gray control rod should be similar to the weight of a black control rod that will be used in the same plant, if both the gray and black control rods have the same interfaces with other components in the reactor. The reactivity worth and weight of a gray control rod can be determined by the material(s) selected and the ultimate configuration of the rod. Typically, the use of a single absorber material does not satisfy both the weight and reactivity worth requirements. Thus, there is room for improvement in GRCA designs for nuclear reactors.